Vacuum heat treating furnaces which employ electrical resistance heating elements are well known. A typical vacuum furnace has a furnace wall and a hot zone chamber of a circular cross-section which houses a series of banks of axial-spaced electrical resistance heating elements suspended from an inner wall of the hot zone chamber by a series of support rods. A heating element is generally made from graphite or molybdenum or a metal alloy, and generates radiant heat in response to electrical current passing therethrough. Popular designs are presented in U.S. Pat. No. 4,559,631 and in my U.S. Pat. No. 4,259,538 (hereafter "the 538 patent").
In the 538 patent I described the problems that arise in connection with operating a vacuum furnace structure which has the insulating material and the heating element, or heating elements, mounted in the heating chamber of the vacuum furnace by a plurality of suitably attached molybdenum rods. The molybdenum rods are conductors of electricity and accordingly must be electrically insulated from the heating element which provides heat (by passing electrical current therethrough) in accordance with its electrical resistance characteristics (I.sup.2 R). It was determined by me at the time of the invention, described and claimed in the above mentioned patent, that electrical insulator devices should be employed to separate the molybdenum mounting rods from the heating element. It was also determined at that time that some of the work piece material evaporates and condenses on the insulator devices to provide a material buildup between the molybdenum rod and the heating element thereby providing a "short circuit". The above mentioned patent teaches the use of molybdenum shields to partially block the space between the molybdenum rod and the electrical insulator device so that no buildup of material can occur therebetween. At the same time said molybdenum shields intercept vaporized work piece material before it condenses on the outer surfaces of the electrical insulator devices. The foregoing described shields have worked out satisfactorily except in certain situations where the temperatures have been sufficiently high and the cycling time sufficiently long, so that the molybdenum shield material, per se, has vaporized and simultaneously the minute amounts of water vapor, (in what would otherwise be a true vacuum), have broken down into hydrogen and oxygen.
It was principally the recognition of this last mentioned phenomenon that led to the conception of the invention set forth in my U.S. Pat. No. 4,425,660, entitled "Shielding Arrangement for a Vacuum Furnace" which in its entirety is incorporated herein by reference. After careful analysis it was determined that under the circumstances of high temperatures and relatively long cycling times, a certain amount of molybdenum from the molybdenum shields was in vapor form and the presence of the oxygen, from the water vapor, acted to oxidize such vaporized molybdenum. It was further determined that the electrical insulator devices have an affinity for molybdenum trioxide (MO.sub.3). It was also discovered that while the molybdenum shields intercepted the vaporized work piece material such shields, per se, provided a buildup of MO.sub.3. On subsequent cycles the MO.sub.3 is reduced to leave molybdenum on the insulator surfaces and such a molybdenum buildup conducts electricity. The invention in U.S. Pat. No. 4,425,660 overcame that problem by providing a pair of graphite shields to be used in place of the molybdenum shields described above. In another embodiment graphite liners were secured to the sides of the above described molybdenum shields, that is to the sides which face the heating element. In yet another embodiment, the graphite liners were secured to the molybdenum shields as described earlier while in addition thereto graphite shields were located on both sides of the heating element facing the shield liners. Even though the graphite might chemically react in a manner similar to that described in connection with the molybdenum, the resulting carbon compounds will not build up on the electrical insulator devices because said electrical insulator devices do not have an affinity for said carbon compounds.
I have now found that under long, high temperature baking cycles with some metals, especially aluminum based materials, even with the improved shields of my U.S. Pat. No. 4,425,660 patent the insulator units are not protected completely from a build-up of conductive material and the problems associated with such buildup. In addition, another problem adding complexity to the solution are described in my co-pending U.S. Patent Applications respectively entitled "Heat Treating Furnace Having Improved Hot Zone" and "Process for Repairing Heat Treating Furnaces and Heating Elements Therefor," both of which were filed May 6, 1999 and are incorporated in their entirety by reference and are continuation-in-part applications of my U.S. application Ser. No. 09/027,868 filed Feb. 23, 1998. In those applications I describe the flexing heating elements are subjected to and sometimes permanent distortions that I have found to occur when such furnaces are put through repeated high temperature and then cooling cycles. The present invention describes new shields for use in high temperature furnaces that accommodate the flexion/distortion problem while substantially reducing the dangers and costs associated with electrical shorting due to conductive chemical deposition on the insulator units.